Pulp purification



Feb. 11, 1958 A. N. PARRETT 2,823,119 PULP PURIFICATION Filed Feb. 2,1952 REFlNING OF SULFITE PULP AT I60 C.

ALPHA CELLULOSLZ (.0

U REFINING YIE KOH SOLUBLE, Z

I- I so 9 a: CD

0 2o 40 so so IOO PERCENT OF TOTAL SODIUM AS SODIUM SULFITE INVENTORflrzfiz/r AfParrzf/ 1 ATTORNE 5 United States Patent PULP PURIFICATIONArthur N. Parrett, Shelton, Wash., assignor to Rayonier Incorporated,Shelton, Wasln, a corporation of Delaware Application February 2, 1952,Serial No. 269,659

6Claims. (CI. 9211) This invention relates to the treatment of wood pulpand has for its object the provision of an improved method of refiningsulfite pulp for the production of high-alpha cellulose. Moreparticularly, the invention provides an improved method ofdigesting-pulp, produced by the acid sulfite digestion of Wood andsubjected to chlorination and washing, with a digestion liquorconsisting principally of sodium carbonate (Na CO and sodium sulfite (NaSO at relatively high temperatures.

It is now common practice in the production of highalpha cellulose tosubject acid sulfite pulp which has been chlorinated and washed todigestion with caustic soda liquor at temperatures usually not in excessof 135 C. and generally below 100 C. My invention produces superior pulphaving better brightness, a higher alpha cellulose content and in betteryield. The pulp is further characterized by better acetylationproperties, as shown by more complete and rapid dissolving inacetylation; and by better properties in the viscose process as shown bygiving yarn or cord of better fatigue properties.

My invention provides a method of subjecting a pulp from the acidsulfite digestion of wood after chlorination and washing to digestionwith an aqueous liquor consisting of sodium carbonate and sodium*sulfite, with or without some sodium hydroxide, in which the sodiumcarbonate is in an amount sufiicient to substantially removehemicellulose so as to produce high-alpha cellulose, and the sodiumsulfite is in an amount sufficient to substantially remove discolorationremaining after the preliminary pulp and chlorination treatments, attemperatures appreciably higher than those used in caustic refiningdigestions, producing a white, high-alpha cellulose in good yield.

In accordance with my invention, I prefer to use a digestion liquor, forrefining acid sulfite pulp, consisting principally of'a mixture ofsodium carbonate and sodium sulfite in which the sodium sulfiterepresents from to 70% of the sodium present in the mixture, at atemperature of at least 140 C. and, in batch operation, most frequentlynot exceeding 185 C., and achieve unpredictable improvements in theyield and quality of the cellulose produced. With less than 10% of thetotal sodium represented by sodium sulfite, the refined cellulose willgenerally lack the desired brightness. With more than 70% of the sodiumrepresented by sodiumsulfite, unsatisfactory alpha purification andundesirably high solubility in the standard vhot potassium test (KOHsolubility) will result. Moreover,.such an excessive proportion ofsodium sulfite will cause difliculties in chemical recovery when theefliuent liquor is furnaced to recover chemicals as a smelt, .mainlybyl'increasing the fusion temperature of the smelt.

It iswell known that digesting sulfite pulp with caustic soda at thehigher than conventional temperatures of this invention is detrimentalrather than beneficial, since.

the additional temperature results in no further increase in alphacellulose, with but slight change in brightness and asignificantly lesseconomical yield.

In the acid sulfite pulping process, a

lignoeellulose 2,823,119 "Patented Feb. 11, 1958 '4 material isgenerally cooked in a solution of sulfurous acid, part of the sulfurousacid being combined as bisulfite. The cation combined thus with thebisulfite ion is generally known as the pulping base and this term isused in this sense herein. Pulping-base cations normally used includecalcium, sodium, ammonium, and magnesium. Pulp produced by acid sulfitedigestion of wood, irrespective of the pulping base, is commonly termedsulfite pulp and the term is used with this meaning herein.

For application of the improved alkaline purification treatment, theprior sulfite digestion of wood is not limited to any particular methodor conditions, such as choice of pulping base. Likewise, the chemicalproperties of the pulp obtained from the sulfite digestion are notcritical and any conventional sulfite digestion treatment may be used.Normally, however, the digestion will be so carried out that a"screenable pulp will be obtained which may be chlorinated withoutun'econo'mical consumption of chlorine. Such sulfite digestionconditions Will normally produce yields of 40% to 50% of unrefined pulp,based on the dry initialwo'od.

Sulfite cooking liquors to produce a screenable pulp will generallycontain about0.'5% to 1.5% of combined sulfur dioxide, representingcombination as neutral sulfite, and 4% to 10% of free sulfur'dioxide.

Similarly, the invention is not limited to the use of any particularwood in the sulfite digestion. Any soft or hardwood which can besatisfactorily pulpedby the acid sulfite process may be used. 'Suitab'lewoods include western hemlock, spruce, southern pine, black gum,birch,'maple and ash.

Following sulfite digestion, the pulp is Washed by any conventional orotherwise convenient manner such as in a blowpit or by drum washers. TheWashed pulp in aqueous suspension is then chlorinated with elementalchlorine according to conventional'practice for sulfite di- Lgested"pulp. The amount of chlorine used is generally remove hemicellulosefrom the unrefined pulp. The color of the resulting pulp, however, wouldnot equal that of pulp from customary sodium hydroxide refining andwould be, in fact, so dark as to be unacceptable. By using a mixture ofsodium carbonate and sodium sulfite, the advantages of a bufferedsolution for removing hernicellulose are retained While the highlycolored materials are simultaneously removed. Presumably this is due toa specific eifect of the sodium sulfite on colored material existing inthe sulfite pulp or formed during the alkaline digestion.

While the digestion operation of the invention with a liquor consistingof sodium carbonate and sodium sulfite gives results superior to thecustomary refining with sodium hydroxide liquor, it is not necessarythat the digestion liquor be entirely free from sodium hydroxide. I mayuse a liquor containing in addition to sodium can bonate and sodiumsulfite, sodium hydroxide in amount representing up to 15% of the sodiumin these sodium compounds as the solution still retains its highlybuffered character and gives the previously described advantages in therefining.

In practice, particularly when the refining is practiced as part of acyclic process of refining and chemical rethe above described eifectivesodium compounds, substantial amounts of sodium sulfate and smalleramounts of other sodium salts such as sodium chloride and sodiumthiosulfate. These additional sodium chemicals when present, arerelatively inert, and their presence is not considered in computing theeffective ranges for the active chemicals, neither as regards the rangeof proportions of chemical composition, nor as regards range of theratio of total amount of active chemicals to pulp.

In carrying out a method of the invention, the chlorinated and Washedacid sulfite pulp is digested in any suitable manner in a liquorcontaining a mixture of sodium carbonate and sodium sulfite, in whichthe sodium sulfite represents from to 70% of the sodium of the mixtureand at a temperature above 140 C., frequently with advantage at atemperature of around 150 to 175 C. With a digestion time of the orderof 45 minutes, optimum results will generally be obtained at 150-175 C.With shorter times, higher temperatures may be used, even above 185 C.The operation may be conducted batchwise or continuously in any suitableequipment. For continuous operation, short times and high temperaturesmay be used.

The consistency of the pulp slurry is not critical but for economicalreasons should be as high as can conveniently be mixed with themechanical equipment available, preferably from 10% to The termconsistency, as used herein, refers to the weight of bone dry pulp pertotal weight of slurry in which it is contained, expressed as percent.

The total amount of sodium carbonate and sodium sulfite used will bedetermined by the characteristics desired in the refined pulp. Usuallyfrom 6% to 20% of sodium salts, calculated as Na O and based on weightof bone dry pulp, will be used, more extensive refining being obtainedwith the higher proportions of chemicals. Since the chemicals mayreadily and advantageously be recovered, as described and claimed in mycopending application Serial Number 271,621, filed Feb. 14, 1952, it iseconomically practical to use higher proportions of chemicals to pulpthan has been possible in past sulfite pulp refining procedures usingsodium hydroxide because the sodium hydroxide could not be recoveredpractically.

After completion of the refining treatment and washing, the pulp may bebleached to a high degree of brightness by any method which will notimpair the high quality of the pulp. In view of the high brightnessresulting from the alkaline refining of the invention, bleach chemicalrequirements will generally be low. For pulps intended for use in eitherthe viscose or acetate processes, bleaching may conveniently be bysodium or calcium hypochlorite. The fullest advantages, however, areobtained by bleaching with aqueous solutions of chlorine dioxide oracidified sodium chlorite. The bleaching may be carried out at eitherhigh or low pulp consistencies and either batchwise or in continuousbleaching equipment.

Pulp refined by the invention, after bleaching, in addition to havingadvantages of a high-alpha cellulose content and high brightness, isgenerally characterized by a low content of impurities Such as pentosansand lignin.

The bleached refined pulp has particular advantages in the viscoseprocess where, as compared with conventionally refined pulp, it givesyarn, cord or film not only of higher quality but also in higher yield.Likewise the bleached refined pulp finds advantageous use in acylationand etherification processes. In acylation processes, especially asregards acetylation, the pulp reacts more rapidly than conventionalhigh-alpha pulps and gives generally clearer solutions.

Other cellulose esters. and ethers for whose preparation the pulp issuitable include cellulose propionate, cellulose aceto-propionate,cellulose aceto-butyrate, ethyl cellulose, methyl cellulose,carboxyrnethyl cellulose, hydroxyethyl cellulose, cellulose sulfate andthe like.

4 The accompanying drawing illustrates graphically the results ofrefining digestions of acid sulfite pulp with liquors containing varyingamounts of sodium carbonate and sodium sulfite.

The following examples and tables illustrate the method and improvementsof the invention:

EXAMPLE I Three batches of chlorinated and washed acid sulfite pulp fromwestern hemlock wood were refined by digesting for 45 minutes in anagitated pot at temperatures of C., C., and C. with liquor equivalent to19.4% Na O (25% NaOH) consisting of a mixture in which one-half the Na owas Na CO and one-half was Na SO Following the refining digestions, thepulp batches were removed, washed with water, and dried.

Alkaline refining would normally be followed by bleaching. A bleachingstep was, however, omitted in these tests in order to show moredefinitely the direct eflect of the alkaline refining.

The refined and dried pulps were analyzed for alpha cellulose and testedfor brightness by the following methods which are similar to those knownand used in the art:

Alpha cellulose:Fraction of pulp insoluble in 18.0% NaOH solution, by amethod closely similar to TAPPI Standard Method T203m.

Brightness:Percent reflectance of light at 457 millimicrons using aBeckman Model D. U. spectrophotometer equipped with reflectanceattachment. Calibration of standards was such as to give results closelyequivalent to those of TAPPI Standard Method T2l7m for measuringbrightness with the General Electric reflection meter.

Using the above refining procedure and conditions and testing methods,the experimental values shown in Table I below were obtained:

Table I Alpha,- Bright- Yield, Temperature Cellulose, ness percentpercent EXAMPLE IA In order to demonstrate the improved acetylationproperties of pulp refined by the invention, the following convenientand rapid laboratory test was used for comparing the acetylationreactivity of samples of pulp fibers:

Small specimens of the pulps to be examined are moistened with distilledwater and dried in a circulating oven at a controlled elevatedtemperature to dry them under comparable conditions. (This step may beomitted if all samples have been previously dried in the same manner.)An accurately weighed sample of 0.5 gram of each pulp is torn into smallbits and placed in a 35 ml. vial. A flattened glass rod is placed in thevial through a hole in the cap and the vial and sample set in a waterbath at 20 C.

The 'acetylating mixture is prepared by mixing 2,500 gms. H 80 88.0 ml.acetic anhydride, and 175.0 ml. acetic acid. This mixture is unstableand should be freshly prepared every two days. 7

To the sample vial in the water bath 15 ml. of the acetylating mixtureare added from a pipette. The pulp and acid are mixed with the glassrod, which remains in the vial. The vials are stored in the Water bathand the mixing repeated every 15-20 minutes. It is important to includea standard sample with each group of unknowns and to handle and agitateall samples alike.

As the pulp samples are acetylated by the mixture they dissolvecontinuously. The time required for substantial solution to take placeand the relative clarity and residual undissolved fibers at the time ofobservation will indicate whether any of the samples is more or lessreactive than the standard. An observation ofcolor is also made.

As an example of pulp refined by the invention, a sample selected fromthe refining experiments described in EX- ample I was tested by theabove test. This sample was the one refined at 150 C. with a 50-50mixture of Na O and Na SO In the above described acetylation test itdissolved in 7 hours to give a clear solution. In comparison, samples ofsulfite pulp refined by conventional of 50-50 sodium sulfite-carbonateequivalent to about 19% Na O.

EXAMPLE IV A sample of chlorinated hemlock sulfite pulp was refined bydigesting it 45 min. at 170 C. in a liquor containing sodium chemicalsin the proportion of 10% Na O methods required 10-12 hours forsubstantial solution and 10 on the p p basis the Sodium Chemicals beingpresent as even then contained many unreacted fiber M 60 55 NagSO 35%,and NaOH 10%. This pulp EXAMPLE II was bleached with sodium hypochloritegiving a product In Table II are shown comprehensive analytical and withthe following analytical properties: Alpha 96.7%, acetylation test dataon a series of samples of western 15 brightness 899%, KQH lh y 3.3%. Thepento an hemlock sulfite pulp refined at 160 C. with sodium ca contentwas low (1.0% asdetermined 'bydistillationwith bonate-sulfite mixturesin amount in each case equivalent hydfoblcmic acid flndprecipitatiol}with bafbitufic 9 o 19. N8 0 (equivalent to 25% NaOH) on the pul Thebleached pulp was made into viscose conta ning The proportions of Na COand Na SO are how 7.5% cellulose and 6.5% NaOH by a conventlonal variedfrom 100% Na CO to 100% Na SO All prostandard procedure and spun into at1re cord usinga portions ar on the ba i f u'ival t f Na o,standardprocedure customarily-used for evaluating pulps. The analyticaland test methods used, in additi t The rayon cords Were tested for theirbreaking strength those already described in Example 1, included thedein both the dry and wet state, their degree of elongation terminationof KOH solubility by measuring the fraction at break in the dry stateand their fatigue life, the last soluble on heating the pulp three hoursat 100 C. in 10% test being made on a machine having a reciprocatingKOI-I solution. motion, with the cord maintained at a temperature of Theresults obtained with the refining test methods of 150 C. and under aload of 1.81 kilograms. These tests Example 11 follow in Table II:resulted in the following results: Tenacity, gm./denier,

Table II Refining Agent, Pct. Acetylation Test of Nero as- RefiningAlpha KOH Bright- Yield, Cellulose, Soluble, ness, Percent- PercentPercent Percent Soln Clarity of Color of NazCO; NBzSOa Time, Soln SolnHrs.

100 0 79. 5 96.8 3. 9 54. 3 6 90 10 7s. 5 96. 3 4. 3 56.5 6 70 so 79. 696.3 5. 3 64. 0 6% 50 79. 6 96. 7 4. 5 68. 4 6% 30 v 85. 0 94. 9 9. 270. 2 6% 10 86. 0 .94. 2 9. 9 74. 4 7 '0 87.8 94. 1 10. 9 76. 1 7

In order toemphasize the eifectsof the varying refining conditions, theabove analyses and tests were made on the refined samples withoutsubsequent bleaching. In actual practice the pulp would be bleachedfollowing refining so that pulp brightness values would be considerablyhigher and colors of the acetates considerably lighter than shown in thetable.

On examination of the analytical data for alpha, brightness, and KOHsolubility, and both dissolving time and color in the acetylation test,it will be seen that the most generally satisfactory pulps were thoserefined within the preferred chemical proportion limits of theinvention, namely with 20 to 70% of the Na O represented by sodiumsulfite.

EXAMPLE III In Table III are given analytical and acetylation test datasimilar to Table II for a series of samples of hemlock sulfite pulprefined at C., in each case with equal parts of Na CO and Na SO in termsof Na O, and with variation of the total amount of sodiumsulfitecarbonate from 7.75% to 25.2% Na O based on pulp.

dry 3.09, wet 1.98; elongation 12.1%; fatigue 302 minutes as comparedwith a fatigue life of 257 minutesfor a cord from conventionally refinedpulp. Moreover, the pulp gave a yield of 98.8% regenerated cellulose ascompared to 96.0% for conventionally refined pulp.

EXAMPLE V A sample of chlorinated sulfite pulp from southern pine wasrefined by digesting it at 16% consistency 45 min. at C. in a liquorcontaining sodium chemicals in the proportion of 12% Na O on the pulpbasis, the sodium chemicals being present as Na CO 55%, Na SO 35%, andNaOH 10%. This pulp was bleached with sodium hypochlorite and thefinished product had the following analytical properties: Alphacellulose 97.5%, brightness 88.7%, KOH solubility 3.6%. The pulp wasalso notable in having a low pentosan content (1.4% as determined bydistillation with hydrobrornic acid and precipitation with barbituricacid).

The pulp was acetylated by a conventional test procedure in which itreacted smoothly to give an acetate of good clarity, color andfilterability.

7 EXAMPLE v1 In order to show what the effect would be in practice ofrecycling efiiuent liquor from the refining of the invention to build upits content of organic materials, a series of successive alkalinerefining digestions was made on separate samples of hemlocksulfite-digested pulp. In each case, the alkaline digestion liquor wasmade up by adding sodium chemicals in the proportions of 55% Na CO 35%Na SO and NaOH. The alkaline digestions were made at 170 C. for 45minutes at a consistency of 10% and using 10% total sodium chemicals asNa O based on the pulps.

In carrying out the series, however, after the first digestion in freshliquor, all the effluent digestion liquor that could be removed from therefined pulp by centrifuging was used in making up the liquor for thenext digestion and similarly through the succeeding digestions. In eachcase, water and chemicals were added to give the required volume ofdigestion liquor containing fresh Na CO Na SO and NaOH in amountequivalent to 10% Na O on the pulp and in the proportions stated. Inaddition, the solutions contained small amounts of residual activechemicals from the liquor recycled from the previous digestion.

Each of the successive digestions provided refined pulp of satisfactoryquality and with no retrogression of quality due to recycling other thana slightly higher bleach demand.

I claim:

1. The method of producing bright high-alpha cellulose which comprisessubjecting chlorinated and Washed pulp from the acid sulfite digestionof wood to digestion in an aqueous liquor consisting principally of amixture of sodium carbonate, sodium sulfite and sodium hydroxide, the 5sodium chemicals varying from 6% to expressed as' Na O based on thepulp, the sodium sulfite in the mixture representing from 10% to 70%. ofthe total sodium, the sodium hydroxide representing not over 15% 'of thetotal sodium and the sodium carbonate representing from to 80% of thetotal sodium, at a temperature in the range of from 150 C. to 175 C. andfor a digestion time of about minutes and at a pulp consistency of from10% to 20% to accomplish the effective diminution of hemicellulose andcolored material.

2. The method of producing high-alpha cellulose in which Wood pulp whichhad been digested with acid sulfite solution, chlorinated and washed, isgiven an alkaline refining treatment which comprises digesting thewashed pulp with an alkaline aqueous solution consisting principally ofa mixture of sodium sulfite and sodium carbonate, said sodium sulfitevarying from 20% to 50% and said soduim carbonate varying from 35% to ofsaid mixture, said solution containing a total of from 6% to 20% of saidchemicals (all said percentages expressed as Na O), said digestion beingcarried out with a pulp consistency of from 10% to 20% and at atemperature of from to 185 C. and for a sutficient time to efiectivelyremove hemicellulose and colored matter thereby producing brighthigh-alpha cellulose in good yield.

3. In the method of claim 2, carrying out the operation with an alkalinesolution having a total of from 50% to 80% of sodium carbonate andsodium hydroxide expressed as Na O.

4. In the method of claim 2, carrying out the digestion at a temperaturein the range of from to C.

5. In the method of claim 2, carrying out the alkaline refining during aperiod of about 45 minutes.

6. In the method of claim 2 said alkaline aqueous solution comprising upto 15% of sodium hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS1,566,118 Rawling Dec. 15, 1925 1,640,853 Richter Aug. 30, 19271,786,890 Braun Dec. 30, 1930 1,802,575 Richter Apr. 28, 1931 1,822,125Blodgett et al. Sept. 8, 1931 1,870,650 Richter Aug. 9, 1932 1,880,046Richter Sept. 27, 1932 1,921,539 Richter Aug. 8, 1933 1,973,557 Bradleyet a1 Sept. 11, 1934 2,249,174 Richter July 15, 1941 2,656,246 Gray etal Oct. 20, 1953 2,694,631 Richter et al. Nov. 16, 1954 FOREIGN PATENTS7,047 Australia Aug. 30, 1927 480,404 Canada Ian. 22, 1952 137,831 GreatBritain May 12, 1921 OTHER REFERENCES Rue et al.: Paper Trade I. (1925),pp. 52 and 53. Rue et al.: Chem. and Met. Eng. (1927), pages 611.Manufacture of Pulp and Paper, 3rd ed., vol. III, sec. 5, pp. 3 and 4(1937), pub]. by McGraw-Hill, New York. Yorston: Dominion Forest ServiceBull. 97, Ottawa, Canada (1942), pp. 32, 39, 41, 46, 48, 53, 55 and 62.

Schelhorn: Paper Trade Journal (1944), pp. 39-44.

U S DEPARTMENT OF COMT'IERCE PATENT OFFICE CERTIFICATE OF CORRECTIONatent No. 2,823 ,119 Arthur N. Parrett February 11, 1958 It is herebycertified that error appeers in the printed specification of the abovenumbered patent requiring correction and that the said Let oers Patentshould read as corrected below.

Co1umn5, line 6, for "Na 0 read Na CO Signed and sealed this 17th day ofJune 1958.

(SEAL) Atteet:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Conmissioner ofPatents

1. THE METHOD OF PRODUCING BRIGHT HIGH-ALPHA CELLULOSE WHICH COMPRISESSUBJECTING CHLORINATED AND WASHED PULP FROM THE ACID SULFITE DIGESTIONOF WOOD TO DIGESTION IN AN AQUEOUS LIQUOR CONSISTING PRINCIPALLY OF AMIXTURE OF SODIUM CARBONATE, SODIUM SULFITE AND SODIUM HYDROXIDE, THESODIUM CHEMICALS VARYING FROM 6% TO 20% EXPRESSED AS NA2O BASED ON THEPULP, THE SODIUM SULFITE IN THE MIXTURE REPRESENTING FROM 10% TO 70% OFTHE TOTAL SODIUM, THE SODIUM HYDROXIDE REPRESENTING NOT OVER 15% OF THETOTAL SODIUM AND THE SODIUM CARBONATE REPRESENTING FROM 35% TO 80% OFTHE TOTAL SODIUM, AT A TEMPERATURE IN THE RANGE OF FROM 150*C. TO 175*C.AND FOR A DIGESTION TIME OF ABOUT 45 MINUTES AND AT A PULP CONSISTENCYOF FROM 10% TO 20% TO ACCOMPLISH THE EFFECTIVE DIMINUTION OFHEMICELLULOSE AND COLORED MATERIAL.